Referring to a magnetic disk device, in some cases in which a sector is subjected to an inter-track interference in data read, ITI (Inter Track Interference) cancel read is carried out to relieve the sector. In order to carry out the ITI cancel read over a plurality of sectors on that occasion, it is necessary to separate a write data path and a read data path in such a manner that these paths function at the same time. For this reason, an encryption circuit and an encryption key holding module are provided separately for the write data path and the read data path in the case in which write data are encrypted and written.